A dispersion consists of a mixture of small solid particulates in a solvent, such as water. The dispersion is said to be a stable colloid if the solid particulates are sufficiently small and homogeneous such that they do not rapidly aggregate and settle from suspension, usually for a period of many days. Such suspensions are often referred to as “colloids” and are useful in many applications. It is the surface properties of the particulates, such as their electrostatic charge, which is responsible for the stability of colloids. Typically the surfaces are significantly charged, positive or negative, so as to provide electrostatic repulsion to overcome forces which would lead to the aggregation and settling of the colloid. In recent years, it has been of interest to surface modify particles, or to “assemble” smaller colloidal particles of opposite charge onto larger core particles to achieve specific properties. However, this is often difficult since the surface modification or assembly disrupts the electrostatic and steric forces necessary for colloidal stability, and stable colloids are not easily obtained, although examples are known. U.S. Pat. No. 5,372,884 describes a cation-modified non-spherical colloidal silica, wherein the cation-modifier is at least one hydrous metal oxide selected from the group consisting of hydrous aluminum oxide, hydrous zirconium oxide and hydrous tin oxide, and the use of the particles in ink-jet media.
It is even more difficult to produce core-shell particles having multiple-shell layers, since aggregation of the particles becomes a limiting factor. Aggregation leads to colloidal instability and may typically be avoided only if very dilute suspensions (less than 1% by weight) are employed. Caruso et al. (J. Amer. Chem Soc. 120, 8523 (1998)) describe a method for preparing nanoparticle-shell multilayers upon larger latex (polystyrene) core particles. A layer-by-layer technique is described in which oppositely charged nanoparticles or polymeric species are sequentially absorbed to the core particle. The technique requires that the core particles be added to a large excess of the shelling polymer or particles and that the unabsorbed fraction (or excess) be removed be repeated centrifugation and wash cycles. Only then is a second shell-layer applied and centrifugation and washing repeated. This method is tedious, requires considerable time and is typically only applicable to dilute (less than about 5 wt %) systems.
Another problem is that multiple-shell, core-shell particles are limited to only a relatively few types of core particles, typically silica particles and organic latexes. This greatly limits the number of chemical structures that can be produced since silica and latex core particles may only be obtained in limited size ranges, and generally only spherical particle shapes are widely available at low cost. Further, the size and shape of such colloids are not well suited to specific applications such as inkjet receivers, described below. There is a need for core-shell colloids having core particles chemically distinct from silica and latex particles, and methods to modify such particles so that core-shell particles having multiple-shells may be produced, having specific particle sizes and particle shapes. The present invention describes cationic, multiple-shell particles comprising a colloidal alumina core. The surfaces of the particles are chemically functionalized with a variety of materials, and the resulting multiple-shell, core-shell particles are stable colloids having high concentrations.
In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of water and an organic material such as a monohydric alcohol, a polyhydric alcohol or mixtures thereof.
An inkjet recording element typically comprises a support having on at least one surface thereof an ink-receiving or image-receiving layer, and includes those intended for reflection viewing, which have an opaque support, and those intended for viewing by transmitted light, which have a transparent support.
An important characteristic of inkjet recording elements is their need to dry quickly after printing. To this end, porous recording elements have been developed which provide nearly instantaneous drying as long as they have sufficient thickness and pore volume to effectively contain the liquid ink. For example, a porous recording element can be manufactured by coating in which a particulate-containing dispersion is applied to a support and is dried. The precise size and shape of the particulates is important since it is desired to achieve both high porosity and high gloss in the coated layer. Large particles (greater than about 500 nm) give coatings with high porosity but low gloss, whereas small particles (less than about 100 nm) give low porosity but high gloss.
When a porous recording element is printed with dye-based inks, the dye molecules penetrate the coating layers. However, there is a problem with such porous recording elements in that the optical densities of images printed thereon are lower than one would like. The lower optical densities are believed to be due to optical scatter which occurs when the dye molecules penetrate too far into the porous layer. Another problem with a porous recording element is that atmospheric gases or other pollutant gases readily penetrate the element and lower the optical density of the printed image causing it to fade.